Ions (Edexcel GCSE Chemistry)

What are ions?

• An ion is an electrically charged atom or group of atoms formed by the loss or gain of electrons
• Negative ions are called anions and form when atoms gain electrons, meaning they have more electrons than protons
• Positive ions are called cations and form when atoms lose electrons, meaning they have more protons than electrons
• All metals lose electrons to other atoms to become positively charged ions
• All non-metals gain electrons from other atoms to become negatively charged ions

Deducing subatomic particles in ions

• An atom is neutral and has no overall charge
• Ions on the other hand have either gained or lost electrons causing them to become charged
• The number of subatomic particles in atoms and ions can be determined given their atomic (proton) number, mass (nucleon) number and charge
• Ions have a different number of electrons to the number of protons, depending on their charge
  • A positively charged ion has lost electrons and therefore has fewer electrons than protons
  • A negatively charged ion has gained electrons and therefore has more electrons than protons

• This loss or gain of electrons takes place to obtain a full outer shell of electrons
• The electronic structure of ions of elements in groups 1, 2, 3, 5, 6 and 7 will be the same as that of a noble gas - such as helium, neon, and argon

Deducing Dot & Cross Diagrams for Ionic Compounds

• Sodium is a group 1 metal so will lose one outer electron to another atom to gain a full outer shell of electrons
• A positive sodium ion with the charge 1+ is formed

Diagram showing the formation of the sodium ion

• Chlorine is a group 7 non-metal so will need to gain an electron to have a full outer shell of electrons
• One electron will be transferred from the outer shell of the sodium atom to the outer shell of the chlorine atom
• A chlorine atom will gain an electron to form a negatively charged chloride ion with a charge of -1

Diagram showing the formation of the chloride ion